This study investigates the physical properties and gas separation performance of a series of PBO-based membranes derived from thermal conversion of blends containing ortho-OH polyamide (PHA) and polyimide (API) precursors. Homogeneous films were obtained in varying PHA/API molar ratios before and after thermal conversion, indicating good compatibility between the two precursors and resulting PBOs. This study reveals that PBO conversion efficiency, fractional free volume and gas transport properties of the resulting PBO-based blend films show an unexpected non-linear dependence on the blend composition (i.e., PHA/API ratio), due to an interplay between API thermal rearrangement (TR) process and PHA cyclodehydration process. Particularly, the B11 blend membrane, containing API and PHA in 1:1 M ratio, showed an optimal combination of gas separation performance and mechanical robustness, while the B31 blend (PHA:API = 3:1) showed little degradation of separation performance relative to both constituent polymers. This is possibly because of the localization of the excess PHA fraction that discourages the formation of inter-connected diffusive transport pathways. In summary, physical blending of API and PHA precursors at an optimal composition is demonstrated to be a simple yet instrumental approach to mitigate the mechanical deterioration issue of API-TR process while greatly improving the separation performance of the PBOs derived from the PHA cyclodehydration process. (C) 2017 Elsevier B.V. All rights reserved.